[IMP] website: improve "Popup" snippet

Review the layout to always use standard bootstrap classes so that
future task targeting modal can target s_popup snippets the same way
(for example: our task allowing to open a popup on click on a button).

This also allows to simplify the css and simplify the options. A new
option also now allows to have a backdrop, or not, so that user clicks
are blocked while the popup is opened, or not.

+ Adapt cookie bar accordingly.

Part of https://github.com/odoo/odoo/pull/52698


task-2210730

Co-authored-by: qsm-odoo <qsm@odoo.com>